Important question: do you get one quick shock when you touch the case, then nothing more as you hold on? Or, do you get a continuous tingling as you hold it?
A single shock at first is most likely to be a static electric discharge. If this is the case, the good news is your computer's case is grounded as it should be. The bad news is you are somehow building up electric charge and some day that will zap an important component and damage it. Find out why you have so much static charge being generated.
If you get a continuous shock as you hang on, there is a serious problem with power connections between the computer, the wall power supply, and some of the peripheral devices plugged into the computer. If this is the case, let us know becasue ther are several steps yous should take to eliminate it.